This invention relates to a method for determining the mininum oil-film thickness of a lubricating oil composition under engine operating conditions using electrical capacitance means.
The hydrodynamic and rheological properties of a lubricant composition and its ability to maintain suitable film thickness for different applications has long been considered by those involved with the theory and application of lubrication technology. While various studies have been made and a number of tests developed for measuring or evaluating film thickness of lubricating compositions with varying degrees of success, most do not satisfactorily address or consider many of the conditions involved in actual use applications, such as fired automotive engines. This can be readily understood, since most test techniques do not involve the squeeze-film conditions experienced in a fired engine. Many of the previous studies of lubricant properties have been based on direct film thickness as used in laboratory bench tests or bearing rigs which do not provide the aforesaid squeeze-film conditions of a fired engine, and indirect measurement in fired engines obtained by monitoring phenomena such as wear, temperature rise, and oil flow.
Several other techniques have been reported for measuring oil-film thickness. Some of these techniques used magnetic sensors or capacitance probes which were placed in, for example, a bearing, and detected the proximity of another surface across the oil film. One such technique is exemplified in SAE Paper 790730 by S. L. Moore entitled "Measurement of the Ring to Linear Oil Film Thickness in a Caterpillar 1-G Diesel Engine," 1979. These techniques generally have the disadvantages of (1) a more complicated change in design to accommodate the probe, (2) interfereing with the normal oil flow in the apparatus and (3) measuring film thickness only at one point, which may not be the point of minimum thickness.
Other studies involving electrical means included that found in SAE Paper 780930 by K. Ninomiya et al entitled "Electrical Observation of Lubricant Film Between a Cam and a Lifter of an OHV Engine," 1978. In this work they were measuring the extent of metallic contact between heavily loaded surfaces. Another technique was disclosed by Fiat involved an engine diagnosis system which used an instrument called a Crankshaft Lubmeter to measure oil-film thickness in bearings by electrical means. In this technique the electrical resistance of the oil-film is measured using somewhat complicated electrical circuitry. The difficulty with measuring the instantaneous electrical resistance of the oil-film is that the surfaces become polarized which increases the apparent resistance of the oil-film and thus leads to error. While this can be avoided by using an AC circuit, it complicates the circuitry even more.
While all of the above methods and apparatus provide some measure for considering lubricant film and the properties thereof under selected conditions, there still remains the need for a simple, relatively direct method in which the film thickness of lubricant compositions can be evaluated when under actual operating conditions.